Direct photons: A nonequilibrium signal of the expanding quark-gluon plasma at RHIC energies

Direct photon production from a longitudinally expanding quark-gluon plasma (QGP) at Relativistic Heavy Ion Collider (RHIC) energies is studied with a real-time kinetic description that is consistently incorporated with hydrodynamics. Within Bjorken's hydrodynamical model, energy nonconserving (anti)quark bremsstrahlung q (\bar{q}) \to q (\bar{q}) \gamma and quark-antiquark annihilation q \bar{q} \to \gamma are shown to be the dominant nonequilibrium effects during the transient lifetime of the QGP. For central Au+Au collisions at RHIC energies \sqrt{s} ~ 200A GeV, we find a significant excess of direct photons in the range of transverse momentum p_T \gtrsim 1.0-1.5 GeV/c as compared to equilibrium results. The transverse momentum distribution at midrapidity falls off with a power law p_T^{-\nu} with 2.5 \lesssim \nu \lesssim 3.0 as a consequence of these off-shell processes, providing a distinct experimental nonequilibrium signature. The rapidity distribution is fairly flat in the interval |y| \lesssim 2. The power law exponent \nu increases with the initial temperature of the QGP and therefore with the total multiplicity rapidity distribution dN_\pi/dy.